This invention relates to methods of inhibiting polymerization of unsaturated monomers, more particularly it relates to methods of inhibiting polymerization of unsaturated carboxylic acid esters and to improved unsaturated carboxylic acid ester compositions. Processes concerning the preparation and the stabilization of unsaturated carboxylic esters are found in Patent Office Class 260, subclass 486.
Substantial quantities of unsaturated carboxylic acid esters, for example methyl methacrylate, are manufactured for use as intermediates in the production of polymers and copolymers. During manufacture, transportation and storage of these esters, it is essential that polymerization of these monomers is avoided. If premature polymerization occurs during manufacture, the polymer fouls or completely plugs production equipment and transfer lines which fouling leads to expensive dismantling and cleaning of production facilities. The problem is particularly acute during the distillation step which step is required to isolate the desired ester monomer. If premature polymerization occurs during transportation or storage, recovery of the polymer from the storage vessel is extremely difficult. Of course, any time that premature polymerization occurs, the yield of the desired monomer is reduced. It is common practice to add stabilizers to unsaturated esters to assure inhibition of polymerization. Preferred polymerization inhibitors not only prevent premature polymerization they do not unduly interfere with the desired polymerization reaction or affect the polymer properties in any way. In polymer production, the effect of polymerization inhibitors is generally reduced by addition of polymerization initiators. Known polymerization inhibitors include phenols (hydroquinone being commonly used) and aliphatic and aromatic amines, for example, phenylenediamines. The subject invention concerns an improved class of phenylenediamine inhibitors.